The decaffeination of green coffee beans is normally carried out by treating the soaked beans with an organic solvent. Commonly chlorinated hydrocarbons are used. In order to obviate the disadvantages inherent therein, it has also been proposed, for example, to use methyl ethyl ketone (Dutch patent application 76 04629). According to another proposal, the beans are not brought into direct contact with the organic solvent. First the beans are extracted with water, whereafter the caffeine is taken up from the aqueous extract by means of the organic solvent (U.S. Pat. No. 2,309,092, to Berry & Walters). Furthermore proposals have been made to avoid the use of organic solvents altogether. According to these proposals, the caffeine from the aqueous solution obtained by the extraction of the beans is taken up in solid adsorbents (Dutch patent application 78.07208). Naturally is it of importance to select such an adsorbent that the other water-solubles present in green coffee are adsorbed in as small a degree as possible. This is important for two reasons. In the first place one would want the beans to lose as little in weight as possible from the treatment. In the second place, the caffeine itself is valuable, and one would of course want to separate it from the mixture with other extracted substances in as simple a way as possible. In connection with the choice of an adsorbent, reference is made to Dutch patent application 78 07208, which describes an adsorbent that has important advantages. Nevertheless, the difficulties are not entirely solved by it.
In the aqueous extract of green coffee beans, chlorogenic acid occurs in a much higher proportion than caffeine. On average, the equilibrium extract of Arabica coffee contains about 10% by weight of caffeine, 45% chlorogenic acid, and 45% "other water solubles". These will hereinafter be termed "residuals". This residue contains in essence sugars. In this connection the term "equilibrium extract" means the following. The beans are first soaked in water to saturation. The soaked beans can be imagined, by way of a model, to consist of an aqueous solution (the equilibrium extract just referred to) taken up in an insoluble residue. The extraction process can be visualized as follows. First, by treating a separate portion of beans, an aqueous solution is made separately, which has the same composition as the equilibrium extract. If this equilibrium extract is contacted with a fresh portion of soaked beans, nothing is extracted from the beans, therefore. If, however, the aqueous solution that has been in contact with these last beans is treated with an adsorbent, certain substances are adsorbed, and if the aqueous extract thus treated is recycled, for example, counter-currently to soaked beans, such substances are extracted from these beans in such quantities as correspond to the quantities taken up in the adsorbent. When an adsorbent such as Duolite S761 is used, as indicated in NL 78 07208, it is found that the adsorbed substances consist approximately of 60% chlorogenic acid, 30% caffeine and 10% "residuals". It is accordingly seen that such an adsorbent has a preferential adsorption capacity for caffeine. Nevertheless, the problems persist that in addition to caffeine other substances are removed from the beans, and that the caffeine needs to be separated from the mixture of substances. All this means that the weight of the beans is reduced as a result of the extraction, that--addition to the disappearance of caffeine from them--the composition of the beans is also changed otherwise, and that the recovery of caffeine still involves considerable difficulties.
It is an object of the present invention to remove caffeine from green coffee beans, if necessary virtually entirely, exclusively by a combination of adsorption and desorption steps, using an adsorbent preferentially adsorbing caffeine from the aqueous equilibrium extract, and whereby the caffeine itself is obtained in a practically pure solution.
In its most generic form, the present invention provides a process for decaffeinating green coffee beans, comprising the following integrated process stages:
(a) contacting the soaked green coffee beans with a caffeine-deficient solution from stage (b), being an equilibrium extract of these beans from which caffeine has been withdrawn, resulting in caffeine-deficient beans on the one hand and an equilibrium extract of the beans, on the other;
(b) contacting the equilibrium extract of the green coffee beans, produced in stage (a), together with the solution of coffee bean components, produced in stage (e), with a caffeine-deficient adsorbent laden with non-caffeine coffee bean components, said adsorbent being produced in stage (c) and having a preferential adsorption capacity for caffeine, resulting, on the one hand, in a caffeine-deficient solution of coffee beans components, which is partially passed to stage (c) and partially recycled to stage (a), and, on the other hand, an adsorbent laden with coffee bean components, which is supplied to stage (e);
(c) contacting the caffeine-deficient solution of coffee bean components produced in stage (b), with "clean" adsorbent, resulting, on the one hand, in a "clean" aqueous solution and, on the other hand, in an adsorbent laden with soluble coffee bean components, but deficient in caffeine, which adsorbent is supplied to stage (b);
(d) contacting the "clean" aqueous solution produced in stage (c) with the adsorbent laden with coffee bean components and rich in caffeine from stage (e) for the desorption of said adsorbent, resulting, on the one hand, in "clean" adsorbent, which is supplied to stage (c) and, on the other hand, a solution of coffee bean components rich in caffeine, which solution is partially discharged and partially supplied to stage (e);
(e) contacting the caffeine-rich solution of coffee bean components from stage (d) with the adsorbent laden with soluble coffee bean components from stage (b), resulting, on the one hand, in a solution of coffee bean components, which is supplied to stage (b), and on the other hand in an adsorbent laden with soluble coffee bean components rich in caffeine, which is supplied to stage (d).
For "soaking" the green coffee beans, as referred to above, these are, as already stated above, kept in contact with water for such a period of time that they no longer absorb water. The term "equilibrium extract" is also defined hereinbefore. Where reference is made to the "laden" solvent, this means not only the adsorbent with the substances adsorbed thereon. The term includes all the material retained by the adsorbent, including those contained in the solution within the pores and between the particles of the adsorbent, which are carried along with the adsorbent mass when the adsorbent is passed in wet condition from one process stage to another.
It is clear that, as the exchange in the various process stages is allowed to proceed more completely, a higher degree of decaffeination is obtained, and also a purer solution of caffeine. This depends, of course, on the nature of the apparatus used for the exchange and the nature of the adsorbent. This will generally be counter-current apparatus with a greater or smaller amount of stages, as will be illustrated hereinafter in the example. If the exchange is caused to be effected thoroughly, the term "caffeine-deficient" in the above should in each case be read as "practically caffeine-free". The term "clean" as used in the above should, in this connection, also have a relative meaning. According as the exchange in the various process steps takes place more completely, so this term is to be construed more strictly.
The selectivity (the degree of preferential adsorption) of an adsorbent to be used must be determined empirically. The adsorbent is brought into equilibrium with solutions of given concentrations of caffeine, chlorogenic acid and residuals. By analysis the ratio is determined of the components in the resin and in the solution being in equilibrium with it. This is done for a variety of concentrations and concentration ratios. From all this a general picture emerges as to the selectivity under the conditions prevailing in the process.
It has been found that the resins of the formophenol type and, as stated before, in particular Duolite S761, have a good selectivity. Duolite S761 is a formophenolic resin produced by polymerizing or copolymerizing monomers with weakly acidic groups, so that the P.sub.Ka of the monomer with the acidic group is higher than about 6.5 P.sub.Ka values.